Titanium dioxide has many uses. As a pigment, titanium dioxide is typically used as a powder, that is, as fine particles having a particle size generally in the range of 100 to 500 nm. It is often used in paper making and in paints, where it can increase brightness and opacity, and is also used in textiles, etc. Though commonly available as a powder/pigment, a coated form of titanium dioxide has been suggested.
For example, U.S. Pat. No. 3,133,893 relates to pigment particles coated with a polymer. The coated pigments are used in coloring plastic fibers. The polymer coating is formed by in situ polymerization. By coating the pigment with a polymer coating, the pigment is thought to be susceptible to forming a stable and fine dispersion within a resin or resin solution without the need for extensive milling. The polymer can be formed from any vinyl monomer and generally is identical or compatible with the composition to be pigmented. Acrylonitrile is used mostly in the examples. Examples 3, 6, and 17 use titanium dioxide as the pigment to be coated.
U.S. Pat. No. 4,771,086 relates to polymer coating of pigment particles for use in paint. The process of coating the pigment particles uses otherwise conventional aqueous-based emulsion polymerization, except that the then typical anionic emulsification and stabilizing agent is replaced with a nonionic stabilizing agent that is sufficient to serve as an emulsifying agent. This change in emulsification agent purportedly results in polymerization exclusively on the surface of the pigment particle and forms a uniform thickness of polymer coating thereon. In contrast, the use of conventional anionic emulsification agents is taught to yield essentially bulk polymerization with a mass having a consistency ranging from cottage cheese to lumpy dough to a sticky plastic and which is reportedly not of any practical value. The resulting coated pigment particles are taught to have “extraordinary” stability against flocculation or settling in comparison with conventional latex systems. Also, the coated pigments are taught to be capable of achieving improved covering power and surface gloss. Titanium dioxide is mentioned and exemplified as a useful pigment to be polymer coated.
U.S. Pat. No. 7,960,026 also relates to a coated pigment for use in paint. The coated pigment is formed by first adhering polymer particles onto the surface of the inorganic pigment to form a “Primary Composite Particle.” This Primary Composite Particle is then subjected to emulsion polymerization to form a polymer layer around the Primary Coated Polymer. The patentees theorized that the polymer particles attached to the inorganic pigment particle serve as seed polymers to promote formation of the polymer layer around the Primary Coated Polymer during the polymerization step. Titanium dioxide is disclosed and exemplified as an inorganic pigment. The composite particles are taught to provide superior covering/opacity and gloss.
It would be desirable to have a paper that could be made at lower cost and/or have superior properties such as opacity, brightness, abrasion and gloss.